Because of the numerous functions they perform during morphogenesis, much of the understanding of TGFB signal transduction stems from studies in developing embryos. However, as these observations rely primarily on examination of TGFI3 ligands and their receptors, a more complete understanding of their role in embryogenesis requires direct observation of when and where TGFB signals are activated. To accomplish this, an antibody has been generated recognizing the active, phosphorylated form of Smads 1, 5, and 8, which transduces signaling by bone morphogenetic proteins (BMPs), a subset of the TGFBs. This antibody will be used to characterize the pattern of Smad/l5/8 phosphorylation in post-gastrula Xenopus embryos. In addition, methods for conditional gene expression will be validated. These will be used to conditionally activate and inhibit endogenous BMP signaling in order examine functional requirements for BMPs after gastrulation. Finally, preliminary observations reveal a domain of Srnadll5/8 phosphorylation in presumptive neural crest cells. Experiments are proposed to address mechanisms by which domains of BMP signaling are established during neural crest specification and functional requirements for BMPs at specific times during neural crest development.